(exclamatory phrase) a playful indication of surprise, misfortune, or insult

popularized by Tracy Morgan of Saturday Night Live, OH SNAP is seemingly derivative of oh no you didn't where an insulted person, for example a guest of Jerry Springer or often a spunky African American woman contends the insult being made against him/her. While derivative of "oh no you di-int," OH SNAP has more of an emphasis on playfulness and can be said by people other than those being insulted.

First known usage was by the 1980's rap song "Just a Friend" by Biz Markie. Its a "rated G" way of saying "oh shit"

So I came to her room and opened the door
Oh, snap! guess what I saw?
A fella tongue-kissin’ my girl in the mouth,
I was so in shock my heart went down south
So please listen to the message that I say
Don’t ever talk to a girl who says she just has a friend